Impact of drought periods on carbon processing across surface-hyporheic interfaces in fluvial systems

Headwater streams essentially link the terrestrial and the aquatic carbon cycle because they transport terrestrial organic and inorganic carbon downstream towards the oceans. However, most of these inputs are processed during this journey. These processes includes down-break of particulate organic matter, transformation and respiration of dissolved organic matter and furthermore, in-stream production of organic matter. In particular, during drought periods the aquatic processes gain importance because terrestrial inputs are diminished. Therefore, carbon cycling in the remaining surface and subsurface flow of the main channel is accelerated and driven by the connectivity of these compartments. As the surface flow ceases carbon processing rates along subsurface flow paths, namely the hyporheic zone, increase. In the light of climate change, longer drought periods, including in currently humid areas, are expected. Within this context, this thesis aims to understand carbon processing across the surface-hyporheic interface of a Mediterranean intermittent stream during a summer drought. Since dissolved organic matter represents the key energy source of aquatic metabolism that ultimately determine in-stream carbon cycling, we focused on the organic matter quantity and quality. We found increasing retention rates of dissolved organic matter along hyporheic flowpaths as water residence time in this compartment increases with the ceasing of surface flow. The evaluation of optical indices of dissolved organic matter quality revealed that the molecular weight decreased and indices related to biological activity increased. Furthermore, we showed that dissolved organic matter from primary production is rapidly respired in the remaining surface water, while humic-like compounds are processed and respired in the hyporheic zone. The dissolved organic matter processing in the hyporheic zone was paralleled with observations of disproportional high partial pressure of CO2 in the interstitial pore water. These CO2 pulses were related to the desiccation of the streambed, as well as dissolved organic matter availability. Our results suggested that the hyporheic zone acts as a humid refuge for microbial activity and that respiration activity immediately restarts when rain events reestablish subsurface flow paths. Associated with this microbial activity, during drought the processing rates of dissolved organic matter, as well as the processing rates of inorganic nutrients were enhanced. Moreover, we explored the effects of a summer drought on subalpine streams by applying different discharge levels in stream-side flumes fed by the water of a subalpine stream. In this experiment, we found high dissolved organic carbon release from in-stream processes in the flumes with the lowest discharges. This dissolved organic carbon release was at the beginning paralleled with a transient increase in gross primary production but continued to rise even when primary production collapsed. While the collapse of primary production might be a consequence of phosphor limitation, respiration and degradation of dissolved and particulate organic matter in the sediment continued throughout. In line with our findings from the Mediterranean stream, this mesocosm experiment highlighted the importance of the hyporheic zone and organic matter stored therein for carbon processing during drought periods. In both study sites, the surface water metabolism was ultimately dominated by respiration, and dissolved organic matter quality of the surface water played an important role on processes in the hyporheic zone. Although the investigated study sites were different in many aspects we found surprising similarities in carbon processing with flow reduction. This suggests that findings from Mediterranean streams might be transferable to other climatic regions under global change scenarios.